1. Field of the Invention
The present invention relates to an apparatus for measuring the flow rate of water vapor in a process gas including steam and, more specifically, to an apparatus which is capable of measuring, with a high level of precision, the flow rate of the steam contained in a gas with a high dew point supplied to, a process reactor such as a fuel cell power generating system when the power generating system is being operated, adjusted or checked.
2. Description of the Related Art
FIG. 2 is a schematic diagram for explaining the principles of a conventional method of measuring a dew point by means of a thermoelectrically cooled, photoelectric dew point meter, the method being known from, e.g., "Handbook of Process Measurement and Control" (FIG. 3.324, published in 1970 by Nikkan Kogyo Shinbun). As shown in FIG. 2, a photoelectric dew point meter for use in process control has a dew point detecting section 10 into which a sample gas is drawn by suction. A mirror 3 made of a metal is disposed inside the section 10. The temperature of the mirror 3, which is automatically held at the dew point of the gas, is continuously measured to indicate the dew point, and the measured dew point is used to perform automatic control. The mirror 3 is cooled by a thermoelectric cooling device 4 employing the Peltier effect and comprising semi-conductor cooling elements. The cooling and the heating of the device 4 are controlled in such a manner as to maintain the temperature of the mirror 3 at the dew point. Specifically, when the humidity of the gas flowing into section 10 changes, the amount of dew on the surface of the mirror 3 increases or decreases. This increase or decrease causes a corresponding increase or decrease in the quantity of light projected from a lamp 1 through a slit 2. This light is then reflected from the mirror 3 and then enters a cadmium sulfide member 6. On the basis of the change in the light quantity, a mirror surface temperature control section 11 comprising an adjuster 8 and a controller 9 operates to either raise or lower the temperature of the mirror 3 so that whenever the dew point changes, the temperature of the mirror 3 is updated to become equal to the new dew point. The temperature of the mirror 3, which is thus held at the dew point, is indicated by a thermometer 5. On the basis of the indicated dew point and on the basis of a certain relation between a gas temperature and saturated water vapor pressure, shown in the following Table 1 (which is the same as Table 3.53 shown in the above-mentioned document), the partial pressure of the water vapor is obtained.
TABLE 1 ______________________________________ SATURATED WATER-VAPOR PRESSURE (mb) (WHEN COEXISTENT WITH WATER) TEMP- ERATURE .degree.C. 0 1 2 8 9 ______________________________________ . . . . . . . . . . . . . . . . . . 0 6.1078 6.5662 7.0547 10.722 11.474 . . . . . . . . . . . . . . . . . . 50 123.40 129.65 136.17 181.53 190.22 . . . . . . . . . . . . . . . . . . 90 701.13 728.19 756.11 943.02 977.61 ______________________________________
The above-described conventional method of measuring the dew point entails the following problems when dew point measurement is performed in a system, such as a fuel cell power generating system, where a gas with a high dew point exists and where the rated temperature is often above 90.degree. C. The dew point measurement performed can be considerably inaccurate because, as will be clearly understood from Table 1, a change of the dew point by 0.1.degree. C. corresponds to a change of the water vapor partial pressure by about 3%. In such systems, therefore, it is difficult to accurately measure the actual flow rate of steam. In addition, when the measured flow rate of the water vapor is used in a feedback arrangement to achieve the correct flow rate of the steam, a manual operation is performed. As a result, the feedback requires a great amount of labor.